In a scanning-type electron microscope, the surface of a specimen is scanned with an electron beam to display an image of the scanned specimen. An ion beam lithography system produces an ion beam for minutely machining the surface of a material. It is necessary for these instruments to focus the charged particle beam, such as an electron beam or ion beam, for minimizing the diameter of the spot of the beam irradiated on the specimen or machined material. This focusing operation is performed by adjusting the magnitude of electromagnetic or electrostatic lenses which act to focus the charged particle beam. Some apparatus of this kind incorporate devices to automate the focusing process. One example of such apparatus is disclosed in U.S. Pat. No. 3,937,959. All the conventional automatic focusing apparatuses operate on the same principle. In particular, such an automatic focusing apparatus detects a signal generated when the surface of a specimen or a material is scanned with a charged particle beam. The resulting signal is variously processed to obtain information regarding the diameter of the cross section, usually in the form of a circle, of the charged particle beam. Accordingly, if this information is incorrect, it is impossible to perform an automatic focusing operation with sufficient accuracy and reliability.
FIG. 1 is a block diagram of main portions of a scanning electron microscope equipped with a conventional automatic focusing mechanism. This microscope includes an electron gun 1 for producing an electron beam 2. That is, the gun 1 is a kind of charged particle beam source. The beam 2 is focused on a specimen 4 by electromagnetic lens (or lenses) 3. At this time, the condition in which the beam is focused is adjusted by varying the output of a lens power supply 5 that controls the magnitude of the lens 3. Under correctly focused conditions, the spot of the beam 2 on the specimen is minimized. Also, the beam 2 is deflected by a deflector 6 consisting of plural sets of coils and other components disposed in the electron beam path. When a deflector power supply 7 supplies a scanning signal to the deflector 6, the electron beam 2 scans the surface of the specimen. When this scan is being made, a secondary electron signal or other signal is generated at every scanned position. The generated signal is converted into an electrical signal by a detector 8 consisting of photomultipliers or the like and then amplified by an amplifier 9. The output signal from the amplifier 9 is used as a video signal for obtaining a scanning image. Note that the configuration used for this purpose is omitted in FIG. 1. Instead, a circuit for performing an automatic focusing operation is shown.
The automatic focusing operation is carried out by a means mainly consisting of a central processing unit 10 that is composed of a computer. When a human operator specifies automatic focusing mode, the central processing unit 10 supplies a control signal to the deflector power supply 7 to establish deflection mode for automatic focusing. In this deflection mode, as shown in FIG. 2, only a certain line segment located in the x-direction on the surface of the specimen 4 which takes a lattice-like form is repeatedly scanned with the beam 2. Each time such a scan is made in the x-direction, the central processing unit 10 varies the magnitude of the lens 3 via a digital-to-analog converter circuit 11 and the lens power supply 5. The output signal from the amplifier 9 is processed as described above. The resulting signal is used to monitor the focused condition of the beam 2 on the specimen. The magnitude of the monitored signal determines whether the magnitude of the lens is increased or reduced.
FIG. 3 shows the waveforms of video signals delivered from the amplifier 9. The intensity of each signal is plotted against time which corresponds to every scanned position on the specimen. Waveform a.sub.1 is obtained when the magnitude of the lens is so adjusted that the beam is correctly focused. Waveform b.sub.1 is obtained when such a correct adjustment is not made. Under the focused condition, the diameter of the cross section of the electron beam 2 is sufficiently small and, therefore, the magnitude of the signal, or the waveform a.sub.1, changes abruptly when it passes across the lattice of the specimen. On the other hand, under a defocused condition, the diameter of the cross section of the beam 2 is large and so the intensity of the signal, or the waveform b.sub.1, changes moderately when it crosses the lattice. The waveforms a.sub.1 and b.sub.1 are passed through a differentiating circuit 12, resulting in waveforms a.sub.2 and b.sub.2, respectively, shown in FIG. 4. When the beam 2 is focused well, the obtained waveform a.sub.2 has high peaks. When the beam is not focused well, the obtained waveform b.sub.2 has low peaks. The peaks of the output signal from the differentiating circuit 12 are detected. The resulting signal can be used to indicate the focused condition of the beam 2. However, since video signals are affected by various noises, if the focused condition of the beam 2 is monitored using only the height of the peaks of the differentiated signal, correct result will not be obtained, and the reliability of the automatic focusing operation will be impaired.
In order to remove these drawbacks, a peak value accumulator 13 is provided. The accumulator accumulates the heights of the peaks of the differentiated signal to produce a signal indicating the focused condition of the beam 2. The deflector power supply 7 produces a blanking signal whenever a scan is made in the x-direction, as well as the scanning signal. The blanking signal is fed to the peak value accumulator 13 as a reset signal and to the central processing unit 10 as a timing signal. The accumulator 13 inverses the peaks of negative polarity of the differentiated signal shown in FIG. 5 to produce waveforms a.sub.3 and b.sub.3. The heights of the peaks of these waveforms are accumulated as waveforms a.sub.4 and b.sub.4 shown in FIG. 6. The output signals h.sub.a and h.sub.b from the peak value accumulator 13 are supplied via an analog-to-digital converter circuit 14 to the central processing unit 10, where the signal is employed to indicate the focused condition of the electron beam 2 on the specimen. This signal derived in this way has the advantage that it is not susceptible to noises. Data indicating various focused conditions of the electron beam which are obtained by supplying various control signals to the lens power supply 5 is stored in the central processing unit 10. The value of the optimum control signal to be applied to the power supply 5 is determined according to the stored data. The optimum condition is maintained to complete the automatic focusing operation.
If the current value of the electron beam impinging on the specimen 4 is kept constant, the amount of signal produced from the specimen may vary greatly because of the composition of the specimen and the shape of the surface. For this reason, as shown in FIG. 4, the waveforms of the signal differ greatly in number of peaks and heights of peaks. Accordingly, under the condition that the electron beam is kept in the optimum focused condition, the value h.sub.a of the signal (shown in FIG. 6) produced from the apparatus shown in Fig. I may vary greatly according to the kind of specimen. As a result, the output signal from the peak value accumulator 13 may become too weak or saturated. At this time, it is impossible to correctly monitor the focused condition of the beam. Consequently, the automatic focusing fails to perform correctly.